Spinach (Spinacia oleracea) is a flowering plant from the Amaranthaceae family that is grown as a vegetable. The consumable parts of spinach are the leaves from the vegetative stage. Spinach is sold loose, bunched, in pre-packed bags, canned, or frozen. There are three basic types of spinach, namely the savoy, semi-savoy and smooth types. Savoy has dark green, crinkly and curly leaves. Flat or smooth leaf spinach has broad, smooth leaves. Semi-savoy is a variety with slightly crinkled leaves. The main market for spinach is baby-leaf. Baby spinach leaves are usually of the flat-leaf variety and usually the harvested leaves are not longer than about eight centimeter. These tender, sweet leaves are sold loose rather than in bunches. They are often used in salads, but can also be lightly cooked.
Downy mildew—in spinach caused by the oomycete fungus Peronospora farinosa f. sp. spinaciae (formerly known as P. effusa)—is a major threat for spinach growers, because it affects the harvested plant parts, namely the leaves. Infection makes the leaves unsuitable for sale and consumption, as it manifests itself phenotypically as yellow lesions on the older leaves, and on the abaxial leaf surface a greyish fungal growth can be observed. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation. The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of ≧80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of P. farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants.
In recent years various resistance genes have been identified that provide spinach plants with a resistance against downy mildew. However, it has been observed that previously resistant spinach cultivars can again become susceptible to the fungus. Investigations revealed that the cultivars themselves had not changed, and that the loss of downy mildew resistance must therefore be due to P. farinosa overcoming the resistance in these spinach cultivars. The downy mildew races (also called physios or isolates) that were able to infect resistant spinach cultivars were collected in a differential reference set, which can be used to test spinach cultivars for resistance. The differential set comprises a series of spinach cultivars (hybrids) that have different resistance patterns to the currently identified pathogenic races.
To date 14 pathogenic races of spinach downy mildew (Pfs) have been officially identified and characterized. Races 4 through 10 have been identified between 1990 and 2009 (Irish et al., 2008, Phytopathol. 98: 894-900), which illustrates the versatility and adaptability of the fungus to overcome resistances in spinach. In different geographical regions different combinations of pathogenic races occur, and the spinach industry therefore has a strong demand for spinach cultivars that are resistant to as many relevant downy mildew races as possible, preferably to all races that may occur in their region, and even to the newest threats that cannot be countered with the resistances that are present in the commercially available spinach cultivars.
In March and August 2011, the “International Working Group on Peronospora farinosa” (IWGP) designated two isolates as the type isolates for new races Pfs12 and Pfs13, respectively. As illustrated by Table 1, these newly identified Peronospora races can break the resistance of many spinach varieties that are currently used commercially worldwide, and they thus pose a serious threat to the productivity of the spinach industry.
Spinach variety Viroflay is susceptible to all known physios, while cultivars such as Lion and Lazio show resistance to multiple races. However, it is crucial to stay at the forefront of developments in this field, as Peronospora continuously develops the ability to break the resistances that are present in commercial spinach varieties. For this reason new resistance genes are very valuable assets, and they form an important research focus in spinach breeding. The goal of spinach breeders is to rapidly develop spinach varieties with a resistance to as many Peronospora races as possible, including the latest identified races, before these races become wide-spread and can threaten the industry.
Recently another new Peronospora isolate has been identified, termed UA4410, which subsequently has been officially named Pfs14. Along with the 13 other officially recognized Pfs races this isolate is publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands.
No single dominant resistance gene is known that confers resistance to the new physios Pfs12, Pfs13 and UA4410. In the absence of a suitable resistance to counter this pathogenic threat, these new isolates may spread during the next growing seasons and cause great damage to the worldwide spinach industry in the immediate future.
In order to confer a resistance that is as broad as possible, i.e. that confers resistance to as many Pfs physios as possible, preferable to all known Pfs physios, it is very useful to be able to stack different resistance genes against Peronospora infection in spinach. Such a combination of different resistance genes on one gene segment is highly desirable. It is much easier if the resistance genes inherit as single dominant loci, because in that case the resistance pattern conferred by the dominant resistance gene cannot segregate away in the progeny of the cross, and will always inherit as one single set of resistances to various pathogenic races.
It is therefore the object of the invention to provide a single, dominant resistance gene in spinach, conferring resistance to various Peronospora races, including the ones that have been most recently identified, which enables the easy transfer of this broad resistance pattern to other spinach plants.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.